Kinetics and mechanism of the reduction-dissolution of cerium oxide catalyzed by fluoride ions in hydrochloric acid solution

Abstract

The dissolution of cerium oxide in aqueous solutions is hindered by the stability of the crystalline network. This reaction can however be catalyzed by fluoride ions and we show here using several metallic fluorides that the cation does not affect the reaction rates and that only the fluoride ions, present as HF under strongly acidic conditions, have a catalytic effect. The dissolution process is shown to consist of two main stages, the first involving the release of Ce4+ ions into the solution under the effect of the fluoride ions, the second, the reduction of the Ce4+ ions by the chloride ions. The experiments show that the first stage of the reaction obeys first order kinetics, the fluoride atoms acting as nucleophiles that replace surface hydroxide functions originating from the superficial protonation of CeO2. The Ce4+ ions are then released as stable CeF22+ complexes. The overall dissolution-reduction mechanism for cerium oxide that we propose on the basis of these results involves first stage on the surface of the grains and a second in the body of the solution.